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At my home (170 metres altitude) it is variable and of course it comes down to how you measure FWHM (FWHM varies considerably within a single image). Winter seeing seems better on average. I've had a good run lately for example.
A sub from tonight has a FWHM 1.9 star but average is more like 2.2 on one sub.
Typically I think 2 on a good night and 2.5 to 3 on an average night and not sure what a bad night number is but I sure know what it looks like at the focuser. You can't focus it as its always shifting.
At my dark site it is typically better than that. The best I have seen is stars of FWHM 1.2 in an AP140 subexposure. Average was 1.7 to 1.9.
Probably around 2.5 is average there perhaps lower. Stars tend not to twinkle above 30 degrees. That's at nearly 800 metres altitude.
Greg.
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